In a large
saucepan, bring the milk and chocolate to a boil over medium heat.
Place the flour in a small glass measuring cup or bowl. Add 2
tablespoons of the hot chocolate milk to the flour, and stir to combine.
Add this mixture back into the soup as it boils. Stir in the sugar and
remove pot from the stove.

Place the egg yolks in a heat-proof
glass measuring cup, and beat lightly with a fork. Add 1/4 cup of the
hot soup to the eggs, and beat vigorously with a whisk to combine. Then
add the egg mixture back into the soup pot, whisk vigorously, and return
the soup to low heat only long enough to warm it.

Pour the soup
through a fine-mesh strainer to catch any bits of coagulated egg. Serve
hot, in small bowls, with a dollop of whipped cream and a bit of grated
chocolate. Or chill the soup for several hours and serve cold.